The english word 'shamrock' comes from the Gaelic 'seamrog' (pronounced sham-rogue) which literally translates as 'young clover'.
While the 4-leaf clover is held as a good luck symbol, the 3-leaf shamrock has a special significance attached to it by the legend of St Patrick.
St Patrick is said to have made use of the little green leaf in his efforts to convert the native Irish Pagan population to Christianity. The 3 leaves helped to explain the concept of the Holy Trinity: one for the Father, one for the Son and one for the Holy Spirit.